Drilling fluid and method for drilling a wellbore

ABSTRACT

A method for drilling a wellbore into a formation. The method includes: providing a mixed metal-viscosified drilling fluid including at least 1% potassium salt and/or at least 0.05% calcium sulfate; circulating the drilling fluid through the well; and drilling into a formation. An anionic thinner may be added if the clay concentration of the drilling fluid reaches a problematic level to adversely affect pumpability of the fluid.

FIELD

This invention relates to methods and fluids used for drilling wells.

BACKGROUND

The process of drilling a hole in the ground for the extraction of anatural resource requires a fluid for removing the cuttings from thewellbore, lubricating and cooling the drill bit, controlling formationpressures and maintaining hole stability.

Many formations present difficulties for drilling, as the formationmaterials which the drilling fluid contacts, can adversely affect theproperties of the drilling fluid.

For example, a fluid that minimizes formation damage and reduces wholemud loss by limiting invasion into the formation and permits easy flowback has been developed, termed herein the mixed metal-viscosifieddrilling fluids including mixed metal oxide (MMO), mixed metal hydroxide(MMH) and combinations of mixed metal oxide and hydroxide (MMOH). Themixed metal-viscosified drilling fluids contain a mixed metalviscosifier, which is an inorganic particle based on magnesium/aluminumoxides and/or hydroxides. The mixed metal particles have a cationiccharacter and react electrostatically with clay particles. Mixedmetal-viscosified drilling fluids include an aqueous-based mixture of atleast one of the mixed metal moieties and an amount of bentonite. Therheology of mixed metal-viscosified drilling fluids limits fluidinvasion into the formation due to high viscosity but the main formationprotection comes from the formation of an external filter cake that iseasy to remove. Simple displacement to water or brine should besufficient for the well to flow back and remove the filter cake.

Unfortunately, however, the rheology of mixed metal-viscosified drillingfluids has broken down when coming into contact with coal finesgenerated from drilling into coal seams, especially young coal. When thedrilling fluid comes in contact with coal fines generated by drillingthrough the seams, the fluid thins, moving toward the rheology of waterand therefore loses many of its beneficial properties. Since coal seamsare, in fact, often considered loss zone formations, and are weak andfriable, the unsuitability of mixed metal-viscosified drilling fluidsfor drilling in coal containing formations is particularly problematic.

In addition to the well known sensitivity of mixed metal-viscosifieddrilling fluids to anionic compounds such as coal, these systems arealso sensitive to the incorporation of reactive drilled clays in so faras such incorporated clays increase the viscosity of the fluid touncontrollable levels and render the system unsuitable. As an example,the clay content of a drilling fluid is measured as an equivalentactivity to the amount of bentonite that is added to the system and istested quantitatively as the Methylene Blue Test (MBT). Increasing theMBT value of a mixed metal-viscosified drilling fluid from 10 pounds perbarrel (ppb) bentonite equivalent (28.6 kg/m³) to 20 ppb bentoniteequivalent (57.2 kg/m³) can render a fluid system unpumpable andunusable.

SUMMARY OF THE INVENTION

In accordance with a broad aspect of the present invention, there isprovided a method for drilling a well through a formation, the methodcomprising: providing a mixed metal-viscosified drilling fluid includingat least 1% w/v potassium salt and/or at least 0.5% w/v calcium sulfate;circulating the drilling fluid through the well; and drilling into theformation.

In accordance with another broad aspect of the present invention, thereis provided a drilling fluid comprising: an aqueous mixture of bentoniteand a mixed metal viscosifier with a pH above about pH 10; and at least1% potassium salt and/or at least 0.5% w/v calcium sulfate.

In accordance with a broad aspect of the present invention, there isprovided a method for drilling a well through a formation, the methodcomprising: providing a mixed metal-viscosified drilling fluid;circulating the drilling fluid through the well while drilling into theformation; identifying a condition of drilling indicative of aproblematic increase in the clay concentration of the drilling fluid;

adding a potassium salt to the drilling fluid to bring the concentrationto at least 1% w/v potassium salt and/or at least 0.5% w/v calciumsulfate; and adding an anionic thinner to the drilling fluid to adjustthe viscosity of the drilling fluid.

It is to be understood that other aspects of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein various embodiments of the invention areshown and described by way of example. As will be realized, theinvention is capable for other and different embodiments and its severaldetails are capable of modification in various other respects, allwithout departing from the spirit and scope of the present invention.Accordingly the detailed description and examples are to be regarded asillustrative in nature and not as restrictive.

DESCRIPTION OF VARIOUS EMBODIMENTS

The detailed description and examples set forth below are intended as adescription of various embodiments of the present invention and are notintended to represent the only embodiments contemplated by the inventor.The detailed description includes specific details for the purpose ofproviding a comprehensive understanding of the present invention.However, it will be apparent to those skilled in the art that thepresent invention may be practiced without these specific details.

Mixed metal-viscosified drilling fluids include a mixed metalviscosifier, which is an inorganic particle based on magnesium/aluminumoxides and/or hydroxides. They are commonly known as mixed metalhydroxides and sometimes referred to as mixed metal oxide (MMO), mixedmetal hydroxide (MMH) and combinations of mixed metal oxide andhydroxide (MMOH). Mixed metal viscosifier, sometimes collectivelyreferred to as MMH, is a mixed metal layered hydroxide compound of thefollowing empirical formula:

M′_(m)M″_(n)(OH)_((2m+3n+qa+br))(A^(q))_(a)(B^(r))_(b) .xH₂O,

where M′ represents at least one divalent metal cation and m is anamount of from greater than zero to about 8; where M″ represents atleast one trivalent metal cation and n is an amount of from greater thanzero to about 6; where A is an anion or negative-valence radical that ismonovalent or polyvalent, and a is an amount of A ions of valence q,provided that if A is monovalent, a is from greater than zero to about8, and if A is polyvalent, a is from greater than zero to about 4; whereB is a second anion or negative-valence radical that is monovalent orpolyvalent, and where b is an amount of B ions of valence r and b isfrom zero to about 4; provided (m+n) is greater than or equal to 1;further provided qa+br cannot be greater than 2m+3n; provided that qacannot equal 2m+3n; and still further provided that (2m+3n+qa+br) isless than 3; and where xH₂O represents excess waters of hydration, withx being zero or more. In certain preferred embodiments (2m+3n+qa+br) isless than 2, more preferably less than 1, and most preferably less than0.5.

While M′ can represent any divalent metal cation of the Groups IA, IIA,VIIB, VIII, IB or IIB of the Periodic Table, preferred divalent cationsare Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn, and more preferred are Mg andCa. M″ is a trivalent metal cation selected from Groups IA or VIII, butpreferred are Al, Ga and Fe, and more preferred is Al.

There must also be present at least one anion or negative-valenceradical, A, and in some cases one (or more) additional anions ornegative-valence radicals, B, may also be present. Examples of theseanions and negative-valence radicals include carbonates, amines, amides,chlorides, oxides, and the like. Preferred therefor are carbonates,oxides and amides.

Alternatively, a combination of materials which can contribute theproportions of constituents of the above empirical formula can beemployed.

One mixed metal viscosifier of interest is the mixed metal hydroxide ofthe formula [Mg_(0.7)Al_(0.3)(OH)₂](OH)_(0.3). Another mixed metalviscosifer of interest is Al/Mg(OH)_(4.7)Cl_(0.3). Mixed metalviscosifiers are commercially available such as from BASF OilfieldPolymers Inc. under the trademark Polyvis™. For example, Polyvis II™ isa mixed metal hydroxide viscosifier.

Until now mixed metal (MMO, MMH and MMOH or collectively MMH)viscosified drilling fluids have been used generally unsuccessfully incoal seams due to the fluid thinning effect from the coal. It isbelieved that the polyanionic nature of coal fines, such as of ligniteand lignosulfonates, interfere with the electrostatic interactions ofthe mixed metal moiety and the bentonite in the drilling fluid,sometimes resulting in a complete collapse of the fluid's rheology.

We have determined that some salts reduce or prevent the thinning effectfrom drilling coals with MMH viscosified fluids. Calcium sulfate and/orpotassium salts including one or more of potassium sulfate, potassiumchloride, potassium acetate and potassium formate may substantiallymaintain the rheology of mixed metal-viscosified drilling fluids whendrilling with coal contaminants. Such salts may add a benefit of shaleswelling inhibition, possibly as a result of the presence of thepotassium ion or calcium ion from the salt.

Potassium sulfate and/or potassium chloride have shown the best resultswith potassium sulfate being particularly preferred.

A wide range of potassium salt concentrations, such as concentrationsgreater than 1% (weight by volume), may be effective in the mixedmetal-viscosified drilling fluid. Generally concentrations of 1-10%(weight by volume) salt and, for example, 1-5% salt (weight by volume)concentrations have been found to be both effective for stabilizing thedrilling fluid against adverse rheological changes due to coalcontamination and advantageous in terms of economics. The amount of saltadded to the drilling fluid may be determined by the amount of coal tobe drilled and/or by the shale reactivity. For example, younger coals,more so than older coals, tend to create greater rheological instabilityfor mixed metal-viscosified drilling fluids and, thus, higherconcentrations (for example greater than 3% and for example 3-10%) ofpotassium salts in the drilling fluid may be useful. Also, if it isdetermined that there are significant coal deposits through which thewell must be drilled, again higher concentrations of potassium salts maybe useful.

For calcium sulfate, concentrations greater than 0.05% (weight byvolume), may be effective in the mixed metal-viscosified drilling fluid.While amounts of up to 5% or more may be used, generally concentrationsof 0.05%-1.0% (weight by volume) calcium sulfate and, for example,0.05-0.5% salt (weight by volume) or 0.1-0.5% concentrations have beenfound to be both effective for stabilizing the drilling fluid againstadverse rheological changes due to coal contamination and advantageousin terms of economics. In younger coals or where significant coaldeposits must be drilled, higher concentrations (for example greaterthan 0.3% and for example 0.3-1.0%) of calcium sulfate in the drillingfluid may be useful. It is believed that the calcium sulfate reachessaturation at about 2 to 3 kg/m3, (0.2 to 0.3% (w/v)), but excessamounts may be added without an adverse effect and in fact may create abuffer of salt to maintain activity, provided the fluid remains a liquidwhich can be circulated through the wellbore. Generally, based on acost/benefit analysis, an upper limit of 1.0% or more likely 0.5% isconsidered sound.

Although the salt may be added after the coal contamination occurs, itis recommended to pre-treat the system for best results. In oneembodiment, for example, the surface hole can be drilled down toapproximately the level of the first coal deposit using any drillingfluid of interest, including for example, prior art mixedmetal-viscosified drilling fluids. When it is determined that the coalseam is close below bottom hole or when the coal seam has been reached,the drilling fluid may be changed over to a drilling fluid according tothe present invention, including a mixed metal-viscosified drillingfluid containing an amount of a potassium salt and/or an amount ofcalcium sulfate.

Alternately, the borehole may be drilled down to and through a coal seamusing a drilling fluid according to the present invention. For example,the entire well substantially from surface, which it will be appreciatedmay include drilling from surface or from below the overburden or afterthe casing point, may be drilled using a drilling fluid according to thepresent invention.

After drilling through the coal seams in the path of the borehole, thepresent drilling fluid may continue to be used for the remainder of thewellbore or other drilling fluids may be used. However, if coal finesmay continue to become entrained in the drilling fluid, for examplewhere a coal seam remains open to contact by the drilling fluid, it maybe useful to continue using the present drilling fluid until drilling iscomplete or the possibility of coal contamination is eliminated. Ifdesired, the drilling fluid returning to the mud tanks at surface may bemonitored to determine the concentration of potassium salt or calciumsulfate therein, as well as other parameters, to ensure that appropriatelevels and fluid characteristics are maintained. For example, any one ormore of the bentonite, mixed metal viscosifier, base, or potassium saltand/or calcium sulfate may be added during drilling to adjust thedrilling fluid parameters. In one embodiment, for example, an amount ofmixed metal viscosifier may be added to the fluid during the course of adrilling operation where reactive formations are drilled and drillcuttings become incorporated to and change the rheology of the drillingfluid. In such a case, the addition of an amount of mixed metalviscosifier can cause the viscosity of the fluid to increase.

As will be appreciated, the drilling fluid may be circulated through thedrill string, drill bit and well bore annulus while drilling.Circulation of the drilling fluid may continue even when drilling isstopped in order to condition the well, prevent string sticking, etc.

During the drilling and circulation, the yield point of the drillingfluid may be maintained above 10 Pa to provide advantageous effects.

Mixed metal-viscosified drilling fluids include bentonite and a mixedmetal viscosifier in water and are pH controlled.

Bentonite is commonly used in drilling fluids and its use will be wellunderstood by those skilled in the art. An untreated bentonite may beparticularly useful. Such a bentonite may be known commercially asuntreated bentonite with a high content of sodium montmorillonite,natural bentonite or untreated Wyoming bentonite.

Generally, mixed metal-viscosified drilling fluids may include lowconcentrations of bentonite (for example, about 15 to 45 kg/m3 or 20 to40 kg/m3 bentonite in fresh water). Sea water-based mixedmetal-viscosified drilling fluids can accommodate more bentonite, aswill be appreciated. Considering that many bentonite based (non-mixedmetal) drilling fluids can contain many multiples more (i.e. two to fourtimes) bentonite than in a mixed metal-viscosified drilling fluid, itcan be appreciated that the viscosity generated using such lowconcentrations of bentonite for mixed metal-viscosified drilling fluidsmight be insufficient for hole cleaning. The addition of mixed metaloxide, mixed metal hydroxide or mixed metal oxide and hydroxide at aweight ratio of 1:8 to 1:12 or 1:9.5 to 1:10.5 to the bentonite producesa stable fluid when the pH is initially maintained above about 10.0 andpossibly between about 10.5 and 13, as may be achieved by addition ofcaustic soda, caustic potash, potassium carbonate and/or soda ash. Oncethe bentonite/mixed metal viscosifier reaction is complete and a gel isformed, it appears that the pH can be lowered to pH 9 or possibly evenlower without any significant loss in viscosity.

In one embodiment, a mixed metal-viscosified drilling fluid may includean aqueous mixture of about 30 kg/m3 bentonite, a mixed metal moiety ina quantity of about 1:10 MMO, MMH or MMOH to bentonite, pH controlled togreater than pH 11 and 1 to 5% potassium salt and/or 0.05 to 1.0%calcium sulphate.

Additives for fluid loss control, lost circulation, etc. may be added tothe drilling fluid mixture, as desired. Non or minor-ionic additives maybe most useful. Some examples may include starch for fluid lossreduction, organophillic lost circulation materials (LCM), etc. Simpletesting may verify the compatibility of any particular additive with thedrilling fluid.

To produce the drilling fluid, the bentonite may first be hydrated inwater. Then the mixed metal moiety is added and pH is adjusted. Thepotassium/calcium salt can be added to the aqueous mixture of bentoniteand mixed metal any time when it is needed for drilling with coalcontamination. Additives such as LCM, fluid loss control agents, etc.can also be added when appropriate, as will be appreciated.

A typical drilling fluid formulation may be according to Table 1.

TABLE 1 A typical drilling fluid according to the invention ProductConcentration Notes Untreated bentonite 30 kg/m3 Prehydrate first infresh water MMH or MMO or MMOH 3 kg/m3 Caustic Soda 0.5 to 1 kg/m3 Tocontrol pH at 11-12.5 Potassium Sulfate 20 to 50 kg/m3 Starch 5 to 10kg/m3

The mixed metal-viscosified drilling fluids described herein are usefulfor successfully drilling into coal and lignite containing formations.Even when contacting coal, such fluids retain their advantageousproperties such as relatively high yield points, high low end rheologyand high and fragile gel strengths. Such properties are advantageous foruse in drilling oil and gas wells, whether vertical, directional orhorizontal due to superior hole cleaning capabilities and because thesefluids mitigate against whole mud fluid losses to formations whether viaformation fractures or high permeability sections.

In particular this new fluid has the advantage over the conventional MMHbased fluids that are very sensitive to the addition or incorporation ofany anionic chemicals or minerals including coal, many drilling fluidadditives that function as thinners such as lignites, humalites,tannins, poly-anionic celluloses, sodium acid pyrophosphate, with achemical formula of Na₂H₂P₂O₇ (SAPP, sometimes identified as adispersant, but acts as a thinner in MMH fluids), or xanthan gum(sometimes identified as a viscosifier, but acts as a thinner in MMHfluids), all of which will cause the rheology of conventionalMMH-bentonite based fluids to collapse and thin.

The addition of salts, such as for example potassium sulfate, insufficient amounts as noted above, prevents the collapse of the uniquevisco-elastic properties of the MMH-bentonite fluids when exposed tocoal or lignite almost completely and it is possible to drill throughcoal seams, even horizontally where significant coal contact may beencountered. The use of such a fluid mitigates against whole fluid lossinto the coal formation, which are typically highly fractured due to theunique rheological properties of the fluid.

As mentioned the rheology of mixed metal-viscosified drilling fluids issensitive to increases in clay content, as may occur when drillingsedimentary formations. As noted above, mixed metal-viscosified drillingfluid systems can only operate within a relatively narrow range ofactive clay concentrations. If such a system does incorporate asignificant amount of water-reactive clays, it will develop aproblematic rheological profile for example a large increase inviscosity.

Even with the present drilling fluid, incorporating high concentrationsof clay causes problematic rheology. In fact, drilling with the presentdrilling fluid through active, young clay zones, with unavoidableincorporation of clay, may increase fluid rheology, such that thedrilling fluid may become substantially unusable (i.e. unpumpable).Addition of an anionic thinner to the present drilling fluid controlsthe rheology and permits continued drilling.

Thus, a method for drilling a well through a formation includes:providing a mixed metal-viscosified drilling fluid; circulating thedrilling fluid through the well while drilling into the formation;identifying a condition of drilling indicative of an increase in theclay content of the drilling fluid; adding a potassium salt to thedrilling fluid to bring the concentration to at least 1% w/v potassiumsalt; and adding an anionic thinner to the drilling fluid to adjust theviscosity of the drilling fluid.

Alternately, 0.05 to 1.0% w/v calcium sulfate may be employed in themethod in place of or in addition to the potassium salt.

Without the addition of a potassium salt or the calcium sulfate, the useof anionic thinners would reduce the viscosity of the mixedmetal-viscosified drilling fluid to nearly that of water.

The formation can be at any depth, any orientation and through any rocktype, such as for example, through carbonates, sandstones, shales, oilshales, etc. The formation can be one known to contain clay orotherwise.

The mixed metal-viscosified drilling fluid can be according to thatdescribed above: an aqueous mixture of a mixed metal viscosifier, asdescribed above, and bentonite, as described above, with pH control, asdescribed above.

The process of identifying a condition of drilling indicative of anincrease in the clay content of the drilling fluid may vary. Forexample, the step of identifying may consider the location of the holebeing drilling, for example using drilling measurements, relative to thelocation of known clay deposits, for example using formation logs. If itis determined that the hole being drilled may, or is going to, passthrough problematic clay deposits, then this can be noted according tothe method and the step of adding an anionic thinner may be initiatedwhen or before the drilling process begins in the clay deposit.Alternately or in addition, fluid rheology can be monitored, theviscosity of the fluid can be measured or the concentration of clay inthe drilling fluid can be monitored directly to identify a conditionindicative of an increase in the clay concentration.

In one embodiment, for example, the methylene blue test (MBT) procedurecan be employed to quantitatively analyze the clay content of thedrilling fluid.

In another embodiment, the fluid viscosity may be monitored as bydetermining the funnel viscosity or more accurately with a device suchas a rheometer, such as a Fann 35 rheometer. When the viscosityincreases beyond an acceptable level, a condition indicative of anincrease in clay content is identified.

The salt (potassium salt and/or calcium sulfate) may be added to thedrilling fluid at any time. For example, the salt may be added duringthe initial production of the drilling fluid, such that the salt ispresent in the system throughout the drilling operation or the salt maybe added only after identifying a condition of drilling indicative of anincrease in the clay content of the drilling fluid. Although the saltmay be added after problematic clay contamination occurs, it isrecommended to pre-treat the system for best results. In one embodiment,for example, the surface hole can be drilled down to approximately thelevel of the first clay deposit using any drilling fluid of interest,including for example, prior art mixed metal-viscosified drillingfluids. When it is determined that a clay deposit is close below bottomhole or when the clay deposit has been reached, the drilling fluid maybe changed over to one including a mixed metal-viscosified drillingfluid containing an amount of a salt. Alternately, if a mixedmetal-viscosified drilling fluid is already being employed, a salt maybe added to the mixed-metal viscosified drilling fluid.

Alternately, the borehole may be drilled down to and into a clay depositusing a mixed metal-viscosified drilling fluid containing greater than1% w/v potassium salt and/or greater than 0.05% calcium sulfate. Forexample, the entire well substantially from surface, which may includedrilling from surface or from below the overburden or after the casingpoint, may be drilled using a drilling fluid including a mixed metalviscosifier, bentonite and the appropriate amount of the salt.

Potassium salts including one or more of potassium sulfate, potassiumchloride, potassium acetate and potassium formate may be useful.Potassium sulfate and/or potassium chloride have shown the best resultswith potassium sulfate being particularly preferred.

A wide range of potassium salt concentrations equal to or greater than1% (weight by volume) may be effective in the mixed metal-viscosifieddrilling fluid. Generally concentrations of 1-10% (weight by volume)salt and, for example, 1-5% salt (weight by volume) concentrations havebeen found to be effective for stabilizing the drilling fluid for theaddition of thinners, while being acceptable in terms of economics. Theamount of salt added to the drilling fluid may be determined by theamount of thinner to be added.

For calcium sulfate, concentrations greater than 0.05% (weight byvolume), may be effective in the mixed metal-viscosified drilling fluid.While amounts of up to 5% or more may be used, generally concentrationsof 0.05%-1.0% (weight by volume) calcium sulfate and, for example,0.05-0.5% salt (weight by volume) or 0.1-0.5% concentrations have beenfound to be both effective for stabilizing the for the addition ofthinners, while being acceptable in terms of economics. The amount ofsalt added to the drilling fluid may be determined by the amount ofthinner to be added.

While the anionic thinner could be added at any time, generally theanionic thinner may be added to the drilling fluid after identifying acondition indicating that the drilling fluid has an increased clayconcentration. For example, the anionic thinner may be added when it isexpected that the wellbore will be drilled into a clay deposit.Normally, however, thinner is added after problematic clay contaminationoccurs. Once the clay concentration or the viscosity indicates aproblematic clay content, the thinner may be added. In one embodiment,for example, thinner is added to address problematic rheologicalprofiles.

The condition indicating that there is a problematic clay content mayvary depending on the equipment and operator's preferences. The fluidmust be pumpable and thinner may be added to ensure that the drillingfluid remains pumpable. In some example embodiments, thinner may beadded as follows:

-   -   a) when funnel viscosity reaches 70 seconds/quart (approximately        equal to 70 s/litre) or possibly when funnel viscosity reaches        60 seconds/quart;    -   b) when the Fann 35 YP reaches 55 Pa to 60 Pa or possibly when        the yield point reaches 50 Pa (at yield point=60 Pa pumping        generally becomes problematic for most rigs);    -   c) using MBT for freshwater based fluids, when the test        indicates clay at ≧20 ppb or possibly ≧13 ppb; or    -   d) using MBT for saltwater based fluids, when the test indicates        clay at ≧40 ppb or possibly ≧25 ppb.

Anionic thinners of interest are anionic chemicals or minerals includingcoal fines, lignite, lignite resin, sulfomethylated lignite,lignosulfonate, humalite, tannin including sulfonated tannin (which isfor example, available as Desco™), sodium asphalt sulfonate,poly-anionic cellulose, penta potassium pyrophosphate (PKPP), sodiumacid pyrophosphate, for example with a chemical formula of Na2H2P2O7(SAPP, sometimes identified as a dispersant, but acts as a thinner inMMH fluids), tetra sodium pyrophosphate (TKPP), sulphomethylatedphenolic resin or xanthan gum (sometimes identified as a viscosifier,but acts as a thinner in MMH fluids). A plurality of these thinners maybe used in combination in some applications.

The above-noted thinners may be added to the circulating drilling fluid.If the thinner is free flowing liquid or powder, it may be addeddirectly.

The thinner is added in an amount sufficient to bring the fluidparameters below the above-noted levels. For example, thinner may beadded and the fluid viscosity monitored and the thinner is added untilthe fluid has a viscosity is reduced to less than YP=60 Pa or in someembodiments below YP=55 Pa or possibly until the yield point is lessthan 50 Pa (for example measured using a Fann 35 rheometer). In anyevent, during the drilling and circulation, the yield point of thedrilling fluid should be maintained above 10 Pa to provide advantageouseffects.

While the actual amounts of thinner used to achieve this above-notedrheological profile will vary depending on the activity of the thinner(i.e. lignite is a less active thinner than sulfonated tannin, andsulfonated tannin is a less active thinner than SAPP), the amount ofclay contamination, etc. Some ranges for example thinners have beenproposed, as follow: lignite may be useful in a range of 0.5 to 20 ppbor more usually 1 to 5 ppb; sulfonated tannin such as methyl ester ofsulfonated tannin (Desco CF™) may be useful in a range of 0.05 to 10 ppbor more usually 0.1 to 5 ppb; and SAPP may be useful in a range of 0.02to 10 ppb or more usually 0.1 to 5 ppb.

After drilling through the one or more clay deposits in the path of theborehole, the present drilling fluid may continue to be used for theremainder of the wellbore or other drilling fluids may be used. However,if clay can continue to become entrained in the drilling fluid, forexample where a clay deposit remains open to contact by the drillingfluid, it may be useful to continue using the present drilling fluiduntil drilling is complete or the possibility of clay contamination iseliminated. If desired, the drilling fluid returning to the mud tanks atsurface may be monitored to determine the concentration of salt andthinner therein, and/or other parameters indicative of problematic claycontent, to ensure that fluid characteristics are maintained. Forexample, any one or more of the bentonite, mixed metal viscosifier,base, salt or anion thinner may be added during drilling to adjust thedrilling fluid parameters. In one embodiment, for example, an amount ofmixed metal viscosifier may be added to the fluid during the course of adrilling operation where reactive formations are drilled and drillcuttings become incorporated to, and change the rheology of, thedrilling fluid. In such a case, the addition of an amount of mixed metalviscosifier can cause the viscosity of the fluid to increase. In anotherembodiment, for example, an initial amount of an anionic thinner andfurther amounts of that or another anionic thinner may be added to thefluid during the course of a drilling operation where reactive clayformations are drilled and clay becomes incorporated to, and changes therheology of, the drilling fluid. In such a case, the addition of anamount of thinner can cause the viscosity of the fluid to decrease.

As noted above, other additives may be employed in the drilling fluidsuch as starch for fluid loss reduction, organophillic lost circulationmaterials (LCM), etc. Simple testing may verify the compatibility of anyparticular additive with the drilling fluid.

To produce the drilling fluid, the bentonite may first be hydrated inwater. Then the mixed metal moiety is added and pH is adjusted. The saltcan be added to the aqueous mixture of bentonite and mixed metal with orbefore the thinner. Additives such as LCM, fluid loss control agents,etc. can also be added when appropriate, as will be appreciated.

The following examples are included for the purposes of illustrationonly, and are not intended to limit the scope of the invention orclaims.

EXAMPLES Example I Drilling Fluids with Coal Contamination

In Example I, drilling fluids were prepared according to the sampledescriptions by hydrating the bentonite, adding the mixed metal moietyand adjusting the pH, as needed. Thereafter, any additives, includingpotassium salt if any, were added.

To simulate coal contamination, lignite was added.

The rheological properties have been tested using a Fann 35 andBrookfield viscometers.

TABLE 2 Composition of Sample #1 Products Sample #1 Untreated Bentonite30 kg/m³ MMH 3 kg/m³ Caustic 0.5 kg/m³ Starch 10 kg/m³

TABLE 3 Results without the addition of Salt Sample #1 + Sample #1 + Mud5 kg/m3 15 kg/m3 Properties Sample #1 Lignite Lignite 600 RPM 86 47 43300 RPM 64 29 25 200 RPM 53 21 18 100 RPM 40 13 10 6 RPM 19 2 1.5 3 RPM17 1 1 10 sec Gel 8 1 0.5 (Pa) PV (mPa*s) 22 18 18 YP (Pa) 21 5.5 3.5LSRV (cP) 54,000 12,000 0 Temperature 22.8 22.3 23.0 (° C.)

TABLE 4 Results using Potassium Chloride Sample #1 + Sample #1 + MudSample #1 + 2% KCl + 5 2% KCl + 15 Properties 2% KCl kg/m3 Lignite kg/m3Lignite 600 RPM 66 47 44 300 RPM 52 31 27 200 RPM 46 23 21 100 RPM 38 1614 6 RPM 18 4 3 3 RPM 16 3 2 10 sec Gel 7 2 1.5 (Pa) PV (mPa*s) 14 16 17YP (Pa) 19 7.5 5 LSRV (cP) 25,000 12,000 9,000 Temperature 21.6 22.122.3 (° C.)

TABLE 5 Results using Potassium Acetate Sample #1 + Sample #1 + Sample#1 + 2% Pot. 2% Pot. Mud 2% Pot. Acetate + 5 Acetate + 15 PropertiesAcetate kg/m3 Lignite kg/m3 Lignite 600 RPM 66 52 48 300 RPM 47 38 35200 RPM 39 32 29 100 RPM 30 25 22 6 RPM 12 10 10 3 RPM 8 8 7 10 sec Gel4 4 4 (Pa) PV (mPa*s) 13 14 13 YP (Pa) 20 12 5.5 LSRV (cP) 31,000 20,00012,000 Temperature 23.2 23.3 23.2 (° C.) Note: Lignite dissolves slower.

TABLE 6 Results using Potassium Formate Sample #1 + Sample #1 + Sample#1 + 2% Pot. 2% Pot. Mud 2% Pot. Formate + 5 Formate + 15 PropertiesFormate kg/m3 Lignite kg/m3 Lignite 600 RPM 66 47 42 300 RPM 53 32 28200 RPM 47 26 22 100 RPM 38 18 16 6 RPM 19 6 5 3 RPM 18 4 4 10 sec Gel 72 2 (Pa) PV (mPa*s) 13 15 14 YP (Pa) 20 8.5 7 LSRV (cP) 21,000 13,00012,000 Temperature 22.1 22.3 22.6 (° C.)

TABLE 7 Results using Calcium Nitrate Sample #1 + Sample #1 + Sample#1 + 2% Calcium 2% Calcium Mud 2% Calcium Nitrate + 5 Nitrate + 15Properties Nitrate kg/m3 Lignite kg/m3 Lignite 600 RPM 60 57 47 300 RPM46 42 34 200 RPM 38 34 28 100 RPM 31 27 22 6 RPM 12 11 7 3 RPM 9 9 5 10sec Gel 5 5 3 (Pa) PV (mPa*s) 14 15 13 YP (Pa) 16 13.5 10.5 LSRV (cP)33,000 23,000 22,000 Temperature 21.5 22.1 22.7 (° C.) Note: Lignitedissolves slower.

TABLE 8 Results using Calcium Chloride Sample #1 + Sample #1 + Sample#1 + 2% Calcium 2% Calcium Mud 2% Calcium Chloride + 5 Chloride + 15Properties Chloride kg/m3 Lignite kg/m3 Lignite 600 RPM 61 51 47 300 RPM44 35 34 200 RPM 36 30 29 100 RPM 27 22 23 6 RPM 10 8 8 3 RPM 8 7 6 10sec Gel 3.5 3.5 3 (Pa) PV (mPa*s) 17 16 13 YP (Pa) 13.5 9.5 10.5 LSRV(cP) 27,000 23,000 22,000 Temperature 24.4 24.4 24.2 (° C.) Note:Lignite dissolves slower.

TABLE 9 Results using Potassium Sulfate Sample #1 + Sample #1 + Sample#1 + 2% Pot. 2% Pot. Mud 2% Pot. Sulfate + 5 Sulfate + 15 PropertiesSulfate kg/m3 Lignite kg/m3 Lignite 600 RPM 75 42 34 300 RPM 60 29 21200 RPM 52 24 16 100 RPM 41 18 11 6 RPM 21 8 2.5 3 RPM 19 7 2 10 sec Gel9 4 2.5 (Pa) PV (mPa*s) 15 13 13 YP (Pa) 22.5 8 4 LSRV (cP) 32,00030,000 25,000 Temperature 24.4 24.0 21.3 (° C.)

TABLE 10 Results using Potassium Chloride Sample #1 + Sample #1 + MudSample #1 + 5% KCl + 5 5% KCl + 15 Properties 5% KCl kg/m3 Lignite kg/m3Lignite 600 RPM 61 52 46 300 RPM 49 39 35 200 RPM 45 35 32 100 RPM 42 3230 6 RPM 16 15 15 3 RPM 12 11 10 10 sec Gel 6 6 5 (Pa) PV (mPa*s) 12 1311 YP (Pa) 18.5 13 12 LSRV (cP) 30,000 18,000 21,000 Temperature 20.120.1 20.1 (° C.)

TABLE 11 Results using Potassium Acetate Sample #1 + Sample #1 + Sample#1 + 5% Pot. 5% Pot. Mud 5% Pot. Acetate + 5 Acetate + 15 PropertiesAcetate kg/m3 Lignite kg/m3 Lignite 600 RPM 63 48 44 300 RPM 55 37 36200 RPM 51 36 34 100 RPM 47 34 32 6 RPM 14 20 16 3 RPM 9 11 11 10 secGel 5 5 6 (Pa) PV (mPa*s) 8 11 8 YP (Pa) 23.5 13 14 LSRV (cP) 27,00014,000 33,000 Temperature 20.1 20.1 20.1 (° C.) Note: Lignite dissolvesslower.

TABLE 12 Results using Potassium Formate Sample #1 + Sample #1 + Sample#1 + 5% Pot. 5% Pot. Mud 5% Pot. Formate + 5 Formate + 15 PropertiesFormate kg/m3 Lignite kg/m3 Lignite 600 RPM 50 46 42 300 RPM 40 33 33200 RPM 37 30 30 100 RPM 32 28 29 6 RPM 9 9 14 3 RPM 5 8 10 10 sec Gel 34 5 (Pa) PV (mPa*s) 10 13 9 YP (Pa) 15 10 12 LSRV (cP) 30,000 29,00031,000 Temperature 20.1 20.1 20.1 (° C.)

TABLE 13 Results using Calcium Nitrate Sample #1 + Sample #1 + Sample#1 + 5% Calcium 5% Calcium Mud 5% Calcium Nitrate + 5 Nitrate + 15Properties Nitrate kg/m3 Lignite kg/m3 Lignite 600 RPM 58 49 44 300 RPM52 42 38 200 RPM 50 41 37 100 RPM 47 35 32 6 RPM 12 11 14 3 RPM 8 8 8 10sec Gel 5 4.5 4.5 (Pa) PV (mPa*s) 6 7 6 YP (Pa) 23 17.5 16 LSRV (cP)35,000 43,000 23,000 Temperature 20.1 20.1 20.1 (° C.) Note: Lignitedissolves slower.

TABLE 14 Results using Calcium Chloride Sample #1 + Sample #1 + Sample#1 + 5% Calcium 5% Calcium Mud 5% Calcium Chloride + 5 Chloride + 15Properties Chloride kg/m3 Lignite kg/m3 Lignite 600 RPM 63 48 43 300 RPM50 37 34 200 RPM 42 34 31 100 RPM 35 29 29 6 RPM 13 12 13 3 RPM 10 9 1110 sec Gel 6.5 6.5 7 (Pa) PV (mPa*s) 13 11 9 YP (Pa) 18.5 13 11.5 LSRV(cP) 40,000 37,000 27,000 Temperature 20.1 20.1 20.1 (° C.) Note:Lignite dissolves slower.

TABLE 15 Results using Potassium Sulfate Sample #1 + Sample #1 + Sample#1 + 5% Pot. 5% Pot. Mud 5% Pot. Sulfate + 5 Sulfate + 15 PropertiesSulfate kg/m3 Lignite kg/m3 Lignite 600 RPM 165  128 91 300 RPM 150  11576 200 RPM 143  109 71 100 RPM 131  100 63 6 RPM 85 67 42 3 RPM 37 58 3910 sec Gel 16 29 22 (Pa) PV (mPa*s) 15 13 15 YP (Pa)   77.5 51 30.5 LSRV(cP) 100,000+    80,000 67,000 Temperature   20.1 20.1 20.1 (° C.)

TABLE 16 Results using Sodium Sulfate Sample #1 + Sample #1 + Sample#1 + 2% Sodium 2% Sodium Mud 2% Sodium Sulfate + 5 Sulfate + 15Properties Sulfate kg/m3 Lignite kg/m3 Lignite 600 RPM 179 39 31 300 RPM155 25 19 200 RPM 143 20 15 100 RPM 123 14 9 6 RPM 72 8 3 3 RPM 63 7 210 sec Gel 31 5 2.5 (Pa) PV (mPa*s) 24 14 13 YP (Pa) 65.5 5.5 4 LSRV(cP) 90,000 50,000 28,000 Temperature 22.0 22.0 22.0 (° C.)

TABLE 17 Results using Sodium Sulfate Sample #1 + Sample #1 + Sample#1 + 5% Sodium 5% Sodium Mud 5% Sodium Sulfate + 5 Sulfate + 15Properties Sulfate kg/m3 Lignite kg/m3 Lignite 600 RPM 207 48 33 300 RPM174 38 22 200 RPM 152 35 18 100 RPM 124 31 13 6 RPM 74 27 11 3 RPM 67 2610 10 sec Gel 28 14 9 (Pa) PV (mPa*s) 33 10 11 YP (Pa) 70.5 14 5.5 LSRV(cP) 100,000 100,000 80,000 Temperature 22.0 22.0 22.0 (° C.)

Example II

Background: Nr Wetaskiwin, Alberta, Drilled 222 mm hole to IntermediateCasing Depth of 1425 mMD and set casing at ˜86.2 degrees inclination inthe Rex Coal formation. Set and cement 177.8 mm casing.

Drilling Fluid: 60 m3 of mud is premixed with the following formulation:30 kg/m3 of natural bentonite is pre-hydrated in fresh water for 16hours. 3 kg/m3 of PolyVis II (MMH) is added over 2 hours. pH is raisedto 12.0 with caustic via chemical barrel over pre-mix tank. Fluidbecomes viscous. 50 kg/m3 of Potassium Sulphate is added.

Drilling in Coal: Intermediate casing shoe and cement are drilled outwith a 156 mm bit using water and then water is displaced over to thepre-mixed system, described above. This well was drilled horizontally inthe Rex Coal formation using the pre-mixed system.

Fluid Properties prior to drilling coal:Premix: 60 m3 circulating system.Depth: 1425 m (87.2 degrees inclination)

Funnel Viscosity: 55 s/L

Mud density: 1050 kg/m3pH: 12.0600 reading: 64300 reading: 61200 reading: 60100 reading: 566 reading: 363 reading: 23PV (mPa·s): 3

YP (Pa): 29 Gels (Pa): 11/11

Filtrate (Fluid Loss, mls/30 min): no control

MBT: 30 Kg/m3

Potassium ion (mg/L): 25,000Fluid properties after drilling to 1451 m in Rex Coal formation:Depth: 1451 m (88 degrees inclination)

Funnel Viscosity: 66 s/L

Mud density: 1060 kg/m3pH: 11.5600 reading: 62300 reading: 55200 reading:—100 reading:—6 reading:—3 reading:—PV (mPa·s): 7

YP (Pa): 24 Gels (Pa): 6/10

Filtrate (Fluid Loss, mls/30 min): 60

MBT: 24 Kg/m3

Potassium ion (mg/L): 22,000

It was determined that the fluid viscosity remained substantially stabledespite drilling pure coal.

Thereafter drilling continued to 1845 m in Rex Coal formation with theaddition of 15×22.7 kg sacks of non-ionic starch (Unitrol Starch) forfluid loss control into 80 m3 system:

Fluid properties at depth 1845 m (91.4 degrees inclination):

Funnel Viscosity: 59 s/L

Mud density: 1050 kg/m3pH: 12.0600 reading: 64300 reading: 56200 reading:—100 reading:—6 reading:—3 reading:—PV (mPa·s): 8

YP (Pa): 24 Gels (Pa): 9/11

Filtrate (Fluid Loss, mls/30 min): 19

MBT: 22 Kg/m3

Potassium ion (mg/L): 20.400

The addition of starch doesn't affect the rheology substantially.

After drilling to 2050 m in the Rex Coal formation the fluid propertieswere as follows (89 m3 system):

Depth: 2050 m (87.8 degrees inclination)

Funnel Viscosity: 85 s/L

Mud density: 1050 kg/m3pH: 12.0600 reading: 80300 reading: 70200 reading: 65100 reading: 606 reading: 473 reading: 44PV (mPa·s): 10

YP (Pa): 30 Gels (Pa): 17/18

Filtrate (Fluid Loss, mls/30 min): 15

MBT: 25 Kg/m3

Potassium ion (mg/L): 22.500

It was determined that a mixed metal viscosified-natural bentonite typerheology can be maintained when drilling through coal with the presentsystem.

Example III The Use of Calcium Sulfate in MMH Drilling Fluids

A bentonite-MMH fluid called Sample #7 was prepared using 30 kg/m³untreated bentonite and 3 kg/m³ MMH (Polyvis II). The method proceededas described above with additions of lignite and calcium sulfate

TABLE 17A Results using calcium sulfate in bentonite - MMH solutionSample #7 + Sample #7 + Mud 0.01 kg/m3 0.04 kg/m3 Property Sample #7Caustic Caustic 600 RPM 91 100 124 300 RPM 80 88 107 200 RPM 74 83 98100 RPM 66 76 86 6 RPM 42 25 28 3 RPM 22 18 20 PV (mPa*s) 11 12 17 YP(Pa) 34.5 38 45 pH 9.2 10.3 10.8 Sample #7 + Sample #7 + 0.01 kg/m3 0.04kg/m3 Mud Sample #7 + Caustic + 20 Caustic + 20 Property 20 kg/m3 Gypkg/m3 Gyp kg/m3 Gyp 600 RPM 61 73 106 300 RPM 53 63 96 200 RPM 48 59 88100 RPM 41 53 78 6 RPM 23 16 26 3 RPM 11 13 21 PV (mPa*s) 8 10 10 YP(Pa) 22.5 26.5 43 pH 8.8 10.2 10.8 Sample #7 + Sample #7 + Sample #7 + 2kg/m3 Gyp + 5 Mud Caustic 2 kg/m3 Gyp kg/m3 Lignite 600 RPM 95 98 93 300RPM 80 91 85 200 RPM 76 89 78 100 RPM 69 81 74 6 RPM 22 24 25 3 RPM 1718 18 PV (mPa*s) 15 7 8 YP (Pa) 32.5 42 38.5 pH 10.7 10.7 10.0 Sample#7 + Sample #7 + Caustic + 5 Caustic + 5 kg/m3 Gyp + 5 Mud kg/m3 Gypkg/m3 Lignite 600 RPM 82 71 300 RPM 72 66 200 RPM 68 60 100 RPM 60 53 6RPM 17 17 3 RPM 14 12 PV (mPa*s) 10 5 YP (Pa) 31 30.5 pH 10.7 9.8

Example IV Drilling Fluids with Clay Contamination

In the following examples, drilling fluids were prepared according tothe sample descriptions by hydrating the bentonite in distilled waterfor at least 16 hours, adding the mixed metal hydroxide moiety andadjusting the pH. Thereafter, any additives, including potassium saltand lignite, if any, were added.

Extra bentonite was added to simulate clay contamination.

The rheology properties were tested using a Fann 35 viscometer.

TABLE 18 Composition of Sample #2 Products Sample #2 Untreated Bentonite 10 ppb (28.6 kg/m3) MMH 1.0 ppb (2.86 kg/m3) Caustic Soda 0.2 ppb (tocontrol pH at 10.0 to 12.0)

TABLE 19 The effect of thinner on drilling fluid with normal amounts ofclay but without potassium salt Mud Sample #2 + Properties Sample #2 1ppb lignite 600 RPM 129 32 300 RPM 102 19.5 200 RPM 87 15 100 RPM 80 9 6RPM 25 1.5 3 RPM 22 1.5 10 Sec Gel 20 2 (lb/100 sq. ft.) 10 Sec Gel 20 2(lb/100 sq. ft.) PV (cpoise) 27 12.5 YP (lb/100 sq. ft.) 75 7

TABLE 20 Composition of Sample #3 Products Sample #3 Untreated Bentonite 14 ppb MMH 1.0 ppb Caustic Soda 0.2 ppb

TABLE 21 The effect of thinner on drilling fluid with higherconcentrations of clay, but without potassium salt Mud Sample #3 +Properties Sample #3 1 ppb lignite 600 RPM >300 45.5 300 RPM 239 29 200RPM 199 22.5 100 RPM 181.5 14 6 RPM 67 2 3 RPM 58 1.5 10 Sec Gel 59 4(lb/100 sq. ft.) 10 Sec Gel 59 7 (lb/100 sq. ft.) PV (cpoise) — 16.5 YP(lb/100 sq.ft.) — 12.5

TABLE 22 Composition of Sample #4 Products Sample #4 Untreated Bentonite 10 ppb MMH 1.0 ppb Caustic Soda 0.2 ppb Non-Ionic Starch 3.5 ppb

TABLE 23 Fluid results with thinner and salt Mud Sample #4 + Sample #4 +Properties Sample #4 1 ppb lignite 3 ppb lignite 600 RPM 172.5 99.5 65.5300 RPM 150.5 70 43 200 RPM 140 57.5 34 100 RPM 128 41 23 6 RPM 99 146.5 3 RPM 98 11.5 5 10 Sec Gel 101 12 5.5 (lb/100 sq. ft.) 10 Sec Gel98.5 15 7 (lb/100 sq. ft.) PV (cpoise) 22 29.5 22.5 YP (lb/100 sq.ft.)128.5 50.5 20.5 Sample #4 + Sample #4 + Sample #4 + 17.5 ppb 17.5 ppb17.5 ppb Potassium Potassium Mud Potassium Sulfate + 1 Sulfate + 3Properties Sulfate ppb lignite ppb lignite 600 RPM 127 103 89.5 300 RPM113.5 84.5 69 200 RPM 106.5 76 59 100 RPM 99 65 46.5 6 RPM 47.5 42.522.5 3 RPM 46 41 21 10 Sec Gel 34 41 23 (lb/100 sq. ft.) 10 Sec Gel 3546 33 (lb/100 sq. ft.) PV (cpoise) 13.5 18.5 19.5 YP (lb/100 sq. ft.)100 66 49.5

TABLE 24 Composition of Sample #5 Products Sample #5 Untreated Bentonite 12 ppb MMH 1.0 ppb Caustic Soda 0.2 ppb Non-Ionic Starch 3.5 ppb

TABLE 25 Fluid results with increased clay content, thinner and salt MudSample #5 + Sample #5 + Properties Sample #5 1 ppb lignite 3 ppb lignite600 RPM 254 121.5 83 300 RPM 229.5 90.5 54 200 RPM 222 74 43 100 RPM 20453.5 29 6 RPM 168 19.5 8 3 RPM 165 15.5 6.5 10 Sec Gel 162 16 7 (lb/100sq. ft.) 10 Sec Gel 164 19.5 8 (lb/100 sq. ft.) PV (cpoise) 24.5 31 29YP (lb/100 sq. ft.) 205 59.5 25 Sample #5 + Sample #5 + Sample #5 + 17.5ppb 17.5 ppb 17.5 ppb Potassium Potassium Mud Potassium Sulfate + 1Sulfate + 3 Properties Sulfate ppb lignite ppb lignite 600 RPM 165 136.5104.5 300 RPM 145 115 76 200 RPM 133.5 105 64 100 RPM 125.5 92.5 49 6RPM 68.5 61.5 24 3 RPM 59.5 59 22 10 Sec Gel 41 62 25 (lb/100 sq. ft.)10 Sec Gel 46 74 40.5 (lb/100 sq. ft.) PV (cpoise) 20 20.5 28.5 YP(lb/100 sq. ft.) 125 94.5 47.5

TABLE 26 Composition of Sample #6 Products Sample #6 Untreated Bentonite 14 ppb MMH 1.0 ppb Caustic Soda 0.2 ppb Non-Ionic Starch 3.5 ppb

TABLE 27 Fluid results with increased clay content, thinner and salt MudSample #6 + Sample #6 + Properties Sample #6 1 ppb lignite 3 ppb lignite600 RPM >300 138.5 96 300 RPM ~300 98 64 200 RPM 295 80 51 100 RPM 28058 35 6 RPM 233 21.5 10 3 RPM 228 17 8 10 Sec Gel 212.5 18 8.5 (lb/100sq. ft.) 10 Sec Gel 209 22.5 9.5 (lb/100 sq. ft.) PV (cpoise) 5-20 (est)40.5 32 YP (lb/100 sq. ft.) 270-290 (est) 57.5 32 Sample #6 + Sample#6 + Sample #6 + 17.5 ppb 17.5 ppb 17.5 ppb Potassium Potassium MudPotassium Sulfate + 1 Sulfate + 3 Properties Sulfate ppb lignite ppblignite 600 RPM 194 157 111 300 RPM 175 132 82 200 RPM 167 120 69.5 100RPM 157.5 105 53 6 RPM 127 73 25 3 RPM 84.5 71.5 23 10 Sec Gel 57 75 27(lb/100 sq. ft.) 10 Sec Gel 59 89 46 (lb/100 sq. ft.) PV (cpoise) 19 2529 YP (lb/100 sq. ft.) 156 107 53

TABLE 28 Fluid properties using potassium chloride Sample #5 + Sample#5 + Sample #5 + 17.5 ppb 17.5 ppb 17.5 ppb Potassium Potassium MudPotassium Chloride + 1 Chloride + 3 Properties Chloride ppb lignite ppblignite 600 RPM 165.5 158 65.5 300 RPM 141.5 140.5 47 200 RPM 133.5 13339 100 RPM 130 121 29.9 6 RPM 70 93 10.5 3 RPM 55.5 88.5 10.5 10 Sec Gel56 82 14 (lb/100 sq. ft.) 10 Sec Gel 63 82 29.5 (lb/100 sq. ft.) PV(cpoise) 24 17.5 18.5 YP (lb/100 sq. ft.) 117.5 123 28.5

Example V

A well was drilled in California. Surface casing (13⅜″) was set at 1935ft, surface casing cement was drilled out with water and then displacedto the drilling fluid. A 311 mm (12¼″) intermediate hole was drilled tointermediate casing point at 1843 m MD (6047 ft) with a MMH-bentonitebased drilling fluid described below. At intermediate casing point 244.5mm (9⅝″) casing was run into the hole and cemented.

Drilling Fluid: 800 bbls (130 m3) of MMH-bentonite based drilling fluidwas pre-mixed with the following specifications: 10 ppb naturalbentonite, 1.0 ppb MMH, pH was raised to 12.0-12.3 and then 17.5 ppbpotassium sulfate and non-ionic starch were added.

Drilling in clay containing formations: The formations immediately belowthe surface casing shoe consisted of a high content of young reactivesmectite clays interspersed in sandy formations and through the drillingprocess some of these solids became incorporated into the drilling fluidsystem. The clay content of the drilling fluid, as measured by themethylene blue test (c.f. API 13), increased quickly from 10 ppb to 20ppb equivalent and initially caused the viscosity of the drilling fluidto increase significantly. The viscosity of the fluid was reduced in acontrollable way with the addition of lignite.

The clay containing horizons were interspersed within sand sectionsuntil approximately 4700 ft (1432 m). Thereafter MBT values did notincrease further and gradually decreased. The well was drilled tointermediate casing point at 6047 ft (1843 mMD) where 9⅝″ casing was setand cemented.

It was determined that the fluid viscosity was kept from increasingbeyond useful levels despite an increase in the clay content to 20 ppb(57 kg/m3) bentonite equivalent with the use of potassium sulfate andlignite. 74 bags of lignite (50 lbs per bag) were added throughout thissection into approximately 1268 bbls (˜200 m3) of circulating drillingfluid.

Fluid loss was controlled with non-ionic starch additions of ˜1.3 ppb3.7 kg/m3).

TABLE 29 Daily fluid parameters of field test Date Day 1 Day 2 Day 5Depth (ft) 1946 4000 6060 Density (ppg) 10.2 10.2 10.2 Funnel Viscosity(s/qt) 34 35 53 pH 12.3 9.5 10.3 600 RPM 21 52 51 300 RPM 17 48 47.5 200RPM 15.5 46 46.5 100 RPM 13.5 44 44.5 6 RPM 8 39 41 3 RPM 8 36 40 10 SecGel (lb/100 sq. ft.) 7 35 38 10 Sec Gel (lb/100 sq. ft.) 7 37 39 PV(cpoise or cP) 4 4 3.5 YP (lb/100 sq. ft.) 13 44 44 Temperature (° C.)57 63 47 MBT (ppb equiv.) 10.5 20 20 Potassium ion (ppm) 36,000 31,00022,500 Fluid Loss (ml/30 min) 24 22 18 Total Volume (bbl) 972 1268 1639Additives - Lignite (lbs) 0 3650 0 Total additives Lignite 0 3650 3650

Example VI

Another well was drilled in California. Surface casing (9⅝″) was set at2015′, surface casing cement was drilled out with water and thendisplaced to the drilling fluid. A 222.2 mm (8¾″) production hole wasdrilled with a MMH-bentonite based drilling fluid. At total depth of3018 mMD (9900 ft) 177.8 mm (7″) casing was run into the hole andcemented.

Drilling Fluid: 700 bbls (110 m3) of MMH-bentonite based drilling fluidwas pre-mixed with the following specifications: 15 ppb naturalbentonite, 1.5-2.0 ppb MMH, pH was raised to 10.0-11.0 and then 40-45ppb potassium sulfate and non-ionic starch were added.

Drilling in clay containing formations: The formations below the surfacecasing shoe consisted of a high content of young reactive smectite claysinterspersed in sandy formations and through the drilling process someof these solids became incorporated into the drilling fluid system. Theclay content of the drilling fluid, as measured by the methylene bluetest, increased to 26 ppb equivalent and initially caused the viscosityof the drilling fluid to increase significantly. The viscosity of thefluid was reduced in a controllable way with the addition of lignite,methyl ester of sulfonated tannin (Desco CF) and SAPP (sodium acidpyrophosphate).

The clay containing horizons were interspersed within sand sections from3600 ft (1097 m) until approximately 8780 ft (2667 m). It was determinedthat the fluid viscosity was kept from increasing beyond usefulnessdespite an increase in the clay content to 26 ppb (74 kg/m3) bentoniteequivalent with the use of potassium sulfate along with methyl ester ofsulfonated tannins, SAPP and lignite. Fluid loss was controlled withnon-ionic starch.

TABLE 30 Daily fluid parameters of field test Date Day 3 Day 4 Day 5Depth (ft) 5800 8214 9900 Density (ppg) 10.4 10.4 11.6 Funnel Viscosity(s/qt) 46 105 71 pH 10.6 10.4 10.4 600 RPM 54 104 98 300 RPM 49 102 87200 RPM 48 101 83 100 RPM 45 97 77 6 RPM 40 80 70 3 RPM 39 51 59 10 SecGel (lb/100 sq. ft.) 39 49 51 10 Sec Gel (lb/100 sq. ft.) 41 52 57 PV(cpoise or cP) 5 2 11 YP (lb/100 sq. ft.) 44 100 76 Temperature (° C.)49 63 74 MBT (ppb equiv.) 16.5 26 27 Potassium ion (ppm) 31,400 36,00032,300 Fluid Loss (ml/30 min) 22 18 12 Total Volume (bbl) 916 1150 1274Additives - Lignite (lbs) 9900 0 550 Total additives Lignite 9900 990010450 Additives - Desco (lbs) 2750 625 500 Total additives Desco 27503375 3875 Additives - SAPP (lbs) 4000 0 0 Total additives SAPP 4000 40004000

It can be seen from the experimental (lab) results and from thecomparison of the lab and the field results that the addition of bothpotassium salt and an anionic thinner (lignite alone or lignite, SAPPand methyl ester of sulfonated tannin) provided a suitable rheologicalprofile of the MMH-bentonite based drilling fluid while experiencing ahigh reactive clay content.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to those embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, but is to beaccorded the full scope consistent with the claims, wherein reference toan element in the singular, such as by use of the article “a” or “an” isnot intended to mean “one and only one” unless specifically so stated,but rather “one or more”. All structural and functional equivalents tothe elements of the various embodiments described throughout thedisclosure that are know or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the elements of theclaims. Moreover, nothing disclosed herein is intended to be dedicatedto the public regardless of whether such disclosure is explicitlyrecited in the claims. No claim element is to be construed under theprovisions of 35 USC 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or “step for”.

I claim:
 1. A method for drilling a well through a formation, the methodcomprising: providing a mixed metal-viscosified drilling fluid;circulating the drilling fluid through the well while drilling into theformation; identifying a condition of drilling indicative of aproblematic increase in the clay concentration of the drilling fluid;adding a potassium salt to the drilling fluid to bring the concentrationto at least 1% w/v potassium salt; and adding an anionic thinner to thedrilling fluid to adjust the viscosity of the drilling fluid.
 2. Themethod of claim 1 wherein providing the mixed metal-viscosified drillingfluid includes providing an aqueous-based drilling fluid including 15 to45 kg/m3 bentonite, mixed metal viscosifier at a weight ratio of 1:8 to1:12, viscosifier to bentonite and a base to maintain the pH above 10.0.3. The method of claim 1 wherein adding a potassium salt occurs duringthe step of providing.
 4. The method of claim 1 wherein adding apotassium salt occurs after identifying.
 5. The method of claim 1wherein adding a potassium salt brings the concentration of potassiumsalt to 1 to 5% w/v.
 6. The method of claim 1 wherein providing themixed metal-viscosified drilling fluid includes: mixing bentonite inwater to form a bentonite mixture; adding a mixed metal viscosifier tothe bentonite mixture; adjusting pH to greater than pH 10; and addingthe potassium salt.
 7. The method of claim 1 further comprising addingany of fluid loss control additives and/or lost circulation materials.8. The method of claim 1 wherein providing the mixed metal-viscosifieddrilling fluid provides a drilling fluid with a yield point greater than10 Pa.
 9. The method of claim 1 wherein adding an ionic thinner isinitiated after identifying a condition of drilling indicative of aproblematic increase in the clay concentration of the drilling fluid.10. The method of claim 1 wherein adding an ionic thinner brings thefluid yield point to between 10 and 60 Pa.
 11. The method of claim 1wherein circulating the drilling fluid is initiated prior to drillinginto a clay deposit.
 12. The method of claim 1 wherein circulating thedrilling fluid is maintained while a clay deposit is open to thedrilling fluid.
 13. The method of claim 1 wherein circulating thedrilling fluid is initiated substantially at surface.
 14. The method ofclaim 1 wherein identifying includes determining the viscosity of thedrilling fluid.
 15. The method of claim 1 wherein identifying includesdetermining the clay concentration in the drilling fluid.
 16. The methodof claim 1 wherein identifying includes considering the location of thewellbore relative to a known location of a clay deposit.
 17. The methodof claim 1 wherein a condition of drilling indicative of a problematicincrease in the concentration of clay includes at least one of: a) afunnel viscosity of 70 seconds/quart; b) a yield point of 60 Pa; c) forfreshwater based fluids, a MBT of ≧20 ppb; or d) for saltwater basedfluids, a MBT ≧40 ppb.
 18. The method of claim 1 wherein the anionicthinner is selected from the group consisting of: coal fines, lignite,lignite resin, sulphomethylated lignite, lignosulfonate, humalite,tannin including sulfonated tannin, sodium asphalt sulfonate,poly-anionic cellulose, penta potassium pyrophosphate (PKPP), sodiumacid pyrophosphate, tetra sodium pyrophosphate, sulphomethylatedphenolic resin or xanthan gum.
 19. The method of claim 1 wherein thepotassium salt is selected from the group consisting of potassiumsulfate, potassium chloride, potassium acetate and potassium formate.20. The method of claim 1 wherein the potassium salt is potassiumsulfate.
 21. The method of claim 1 wherein the potassium salt ispotassium chloride.
 22. The method of claim 6 wherein the pH is adjustedusing caustic soda, caustic potash, potassium carbonate or soda ash. 23.A method for drilling a well through a formation, the method comprising:providing a mixed metal-viscosified drilling fluid; circulating thedrilling fluid through the well while drilling into the formation;identifying a condition of drilling indicative of a problematic increasein the clay concentration of the drilling fluid; adding calcium sulfateto the drilling fluid to bring the concentration to at least 0.05% w/vcalcium sulfate in the drilling fluid; and adding an anionic thinner tothe drilling fluid to adjust the viscosity of the drilling fluid. 24.The method of claim 23 wherein providing the mixed metal-viscosifieddrilling fluid includes providing an aqueous-based drilling fluidincluding 15 to 45 kg/m3 bentonite, mixed metal viscosifier at a weightratio of 1:8 to 1:12, viscosifier to bentonite and a base to maintainthe pH above 10.0.
 25. The method of claim 23 wherein adding a calciumsulfate occurs during the step of providing.
 26. The method of claim 23wherein adding a calcium sulfate occurs after identifying.
 27. Themethod of claim 23 wherein adding a calcium sulfate brings theconcentration of calcium sulfate to 0.05 to 1.0% w/v.
 28. The method ofclaim 23 wherein providing the mixed metal-viscosified drilling fluidincludes: mixing bentonite in water to form a bentonite mixture; addinga mixed metal viscosifier to the bentonite mixture; adjusting pH togreater than pH 10; and adding the calcium sulfate.
 29. The method ofclaim 23 further comprising adding any of fluid loss control additivesand/or lost circulation materials.
 30. The method of claim 23 whereinproviding the mixed metal-viscosified drilling fluid provides a drillingfluid with a yield point greater than 10 Pa.
 31. The method of claim 23wherein adding an ionic thinner is initiated after identifying acondition of drilling indicative of a problematic increase in the clayconcentration of the drilling fluid.
 32. The method of claim 23 whereinadding an ionic thinner brings the fluid yield point to between 10 and60 Pa.
 33. The method of claim 23 wherein circulating the drilling fluidis initiated prior to drilling into a clay deposit.
 34. The method ofclaim 23 wherein circulating the drilling fluid is maintained while aclay deposit is open to the drilling fluid.
 35. The method of claim 23wherein circulating the drilling fluid is initiated substantially atsurface.
 36. The method of claim 23 wherein identifying includesdetermining the viscosity of the drilling fluid.
 37. The method of claim23 wherein identifying includes determining the clay concentration inthe drilling fluid.
 38. The method of claim 23 wherein identifyingincludes considering the location of the wellbore relative to a knownlocation of a clay deposit.
 39. The method of claim 23 wherein acondition of drilling indicative of a problematic increase in theconcentration of clay includes at least one of: a) a funnel viscosity of70 seconds/quart; b) a yield point of 60 Pa; c) for freshwater basedfluids, a MBT of ≧20 ppb; or d) for saltwater based fluids, a MBT ≧40ppb.
 40. The method of claim 23 wherein the anionic thinner is selectedfrom the group consisting of: coal fines, lignite, lignite resin,sulphomethylated lignite, lignosulfonate, humalite, tannin includingsulfonated tannin, sodium asphalt sulfonate, poly-anionic cellulose,penta potassium pyrophosphate (PKPP), sodium acid pyrophosphate, tetrasodium pyrophosphate, sulphomethylated phenolic resin or xanthan gum.41. The method of claim 39 wherein the pH is adjusted using causticsoda, caustic potash, potassium carbonate or soda ash.